Kaolin clay pigments are obtained from kaolin. Kaolin, also known as china clay, is a type of rock that contains kaolin minerals. Kaolin minerals are hydrous alumina silicate clay minerals that include the minerals kaolinite, dickite, nacrite and halloysite. Primary kaolin deposits are formed in situ by weathering or hydrothermal alteration of feldspar or mica minerals contained in igneous or metamorphic rocks to kaolin minerals. Sedimentary kaolin deposits contain kaolin mineral-bearing sediments that were eroded and transported from a primary kaolin source rock, kaolin minerals resulting from the post-depositional alteration of feldspar- and mica-bearing sediments, or both. Commercially mined kaolins can contain some impurities such as clay-sized ferruginous or titaniferous impurities that impart an undesirable color to the kaolin clay. Some kaolin deposits contain impurities such as clay minerals of the smectite group that have an undesirable effect on the rheology of the kaolin. Most commercial kaolin deposits have coarse-grained sand to silt size impurities called "grit" that are generally above 45 microns. Grit particle impurities in a kaolin clay pigment used for paper coating may cause imperfections on the coated paper surface that resemble scratches or cause excessive wear of the paper, coater blade by abrasion of metal surfaces in contact with the grit-bearing pigment.
The most common kaolin mineral is a naturally occurring hydrous alumina silicate known as kaolinite (Al.sub.2 Si.sub.2 O.sub.5 (OH).sub.4). Kaolinite is the most common mineral in the kaolin clay widely used in the paper industry as fillers and/or coating pigments. Kaolin clays used for pigments are also called china clay or hydrous kaolin. Kaolinite particles occur over a range of sizes and a range of aspect ratios. The aspect ratio for kaolinite is defined as the diameter of a kaolin particle divided by its thickness. Thus, a kaolin deposit mined commercially will generally not contain particles of a single size, such as, for example, particles all of which are 2 microns. Typically after minimum refining, a degritted kaolin will contain particles ranging in size from submicron or colloidal to particles 45 micrometers or larger.
Kaolin from different deposits, or even from different parts of the same deposit, can vary widely in the content of impurities, particle size distribution, as well as shape, or aspect ratio, of the kaolinite particles. In general, kaolinite particles smaller than about 1 micrometer occur as individual platelets. As the size of the kaolinite particles increase from about 1 micrometer, a larger proportion of the kaolinite particles occur as stacks or booklets of platelets as opposed to discrete individual platelets. Particle sizes of kaolin clay are conventionally determined by sedimentation using Stokes Law to convert settling rates to particle size distribution, and assume a spherical particle shape for the kaolin particles, hence, the use of the conventional term "equivalent spherical diameter (e.s.d.)" to designate particle size.
Kaolin clay pigments are widely used to coat and to fill paper products. It was formerly the practice to simply use relatively coarser size kaolins to fill papers and to employ finer size kaolins to coat paper. In paper, the coarser kaolin fillers functioned primarily as a pulp extender. When used to coat paper, the finer kaolin pigments improve brightness of the paper, provide a smooth, ink-receptive surface, and improve gloss with improved print quality and aesthetic appearance.
Kaolinite stacks or booklets can be mechanically delaminated. Delamination is generally known as the process of splitting kaolinite stacks or booklets, along a basal 001 cleavage or parting plane, into discrete platelets. One delaminating operation involves subjecting the naturally occurring kaolin stacks to shearing forces in an extruder, thereby reducing the kaolin stacks to discrete platelets. Another operation involves subjecting the naturally occurring kaolin stacks in an aqueous clay slurry to the cleaving or delaminating action of an attrition mill or a sand grinder. Reference may be made to U.S. Pat. No. 3,615,806 of Andrew Torock and Thomas F. Walsh for a thorough discussion of the process of delamination of kaolin clay. The kaolin pigments which have been delaminated can be used in paper coating to improve the opacity as well as enhance the smoothness of the paper surface. See for example, U.S. Pat. No. 3,171,718 to Gunn et al.
Kaolin clay can also be thermally structured through a calcination process which irreversibly converts the kaolinite into a material called "metakaolinite". Calcination causes the kaolin particles to stick or fuse together into porous aggregates and results in a pigment with higher light scattering caused by a higher index of refraction and a higher amount of light scattering surface than a kaolin which is not calcined.
Kaolin clay, hydrous kaolin, (Al.sub.2 Si.sub.2 O.sub.5 (OH).sub.4) is white in color, has a fine particle size, and is relatively chemically inert, and, in addition to its low cost, makes it an ideal paper filler. Calcined (anhydrous) kaolin is also available for use as a filler and can impart greater opacity to paper than the hydrous kaolin. However, calcined kaolin has the serious disadvantage of being more abrasive than hydrous kaolin clays. Hydrous and calcined kaolin clays are used to coat paper. However, calcined kaolin is generally used as a minor blend component in a coating pigment formulation because of its higher cost, poor glossing properties, and higher abrasion.
The purpose of paper coating is to cover an irregular paper surface comprised of cellulose wood fiber with a pigment-binder formulation, that when dry, leaves a smooth and brighter surface ready for printing. It is common practice to use kaolin clay along with other mineral pigments, such as titanium dioxide and calcium carbonates, as a coating in a coating formulation comprising starch and/or latex.
U.S. Pat. No. 4,241,142 to Kaliski, et. al. and assigned to Engelhard Minerals and Chemicals Corporation, discloses a novel clay pigment used in the production of dull and matte-finished coated printing papers. The novel clay pigment comprises a mixture of a coarse-size fraction of naturally-occurring kaolin clay containing a substantial proportion of kaolinite booklets in the particle size range of 2 to 5 microns e.s.d., and mechanically delaminated kaolin platelets in the particle size range of 2 to 10 microns.
A paper coating pigment comprising mechanically delaminated kaolin particles is disclosed in U.S. Pat. No. 5,169,443 to Willis, et al. and assigned to Engelhard Corporation. This pigment possesses the opacification, smoothness and printability advantages of conventional delaminated kaolin pigments but have desirably low viscosity and gloss not characteristic of conventional delaminated kaolin pigment. This pigment is designed for gravure and offset printed light-weight coated groundwood paper.
U.S. Pat. No. 2,992,936 to Rowland and assigned to Georgia Kaolin Company discloses an undelaminated and defined kaolin clay pigment with 10% to 15% of particles below 0.3 microns that provides improved brightness, opacity and gloss on coated paper.
U.S. Pat. No. 5,085,707 to Bundy et al. and assigned to Georgia Kaolin Company discloses a delaminated and defined kaolin clay pigment and its blend with a delaminated, No. 1 coating and No. 2 coating clays to obtain superior opacity, brightness and gloss on coated paper. This disclosure describes a standard engineered kaolin pigment sold commercially today.
Some coating formulations involve a combination of different types of kaolin pigments, such as a certain percentage of calcined kaolin clay and a certain percentage of hydrous kaolin clay, which combination can result in "poor" dispersion of particles in that the two types of kaolin clay pigments may aggregate, which may disrupt the coating structure, resulting in poor sheet optics, poor printability, or both. Additionally, many paper coating compositions comprise an abundance of fines, which tend to affect the optical properties of the sheet by improving gloss at the expense of sheet brightness for some grades and opacity.
Finally, kaolin clay pigments for coating are commonly shipped to paper mills as slurries by railroad tank cars, tanker trucks or in tank compartments on ships or barges. The shippable solids of a kaolin clay pigment slurry is dependent upon its fluidity at high solid levels both in terms of its low-shear viscosity that relates to how the slurry flows out of a tank and its high-shear viscosity that relates to how the slurry remains fluid as it passes through pumps and under the blade of the paper coating machine. Engineered pigments for coated freesheet applications such as Astra-Plus.TM. generally has a shippable pigment-water slurry solids close to 65%. Delaminated kaolin coating clays typically have a shippable pigment-water slurry solids below 68%. Standard No. 1 and No. 2 coating clays typically have a shippable pigment-water slurry solids of 70% or higher. There is an economic incentive to ship kaolin clay pigments at the highest possible solids to reduce the cost of shipping water with the pigment. Also, paper coating formulations for the coated freesheet market generally have a restrictive water balance to maintain quality and cost without adding expensive thickening agents into the formulation. It is also commonly theorized that higher coating solids improves surface coverage, that is the ability for pigment particles to cover the network of wood fibers and pores that form the surface of the paper basesheet. Therefore, paper coaters prefer pigment slurries shipped at the highest possible solids.
There is, therefore, a need in the industry for a fluid, low viscosity coating pigment for paper, particularly for wood free or free sheet paper, which provides for a highly glossy surface with good opacity and sheet brightness.